1. Field of the Invention
The present invention relates to a method of adjusting a clamping force in a servo-controlled welding gun adapted to perform a desired welding operation on a workpiece held in a clamped manner on the electric resistance spot welding gun controlled by a servo motor.
2. Description of Related Art
In a known electric resistance spot welding gun, a workpiece is clamped between one electrode (a movable or upper electrode) driven by an air cylinder or a servo motor for example and the other electrode (a static or lower electrode) and a large current is passed between these electrodes for a desired welding work. If, as disclosed in U.S. Pat. No. 5,321,225, a servo motor is used for the driving source to operate the movable electrode, the clamping force applied to the welding point of a workpiece held between the movable and static electrodes is generally dependent on a current supplied to the servo motor. Therefore, calculating beforehand supply currents to the servo motor necessary for generating a plurality of clamping forces and storing the calculated clamping forces and supply currents as a clamping force vs. electric current table in the welding gun controller. A desired clamping force at a welding operation is provided by selecting a supply current to the servo motor corresponding to a specified clamping force.
Generally, the clamping force vs. electric current table can be theoretically obtained based on various electrical and mechanical data of the servo motor and welding gun. However, since the relationship between the obtained clamping forces and electric currents are of theoretical, it becomes somewhat different from the relationship at an actual clamping operation. This difference or error is caused by an error in fabrication of the servo motor and the welding gun, an error due to a dynamic element caused by an actual clamping operation caused by friction and rigidity of movable sections, and an error due to aging, or abnormal condition caused by time, of the apparatus for example. Therefore, it is necessary, before performing a welding operation, to actually clamping a workpiece between the electrodes of the welding gun, measure the clamping force at this moment with a clamping force measurer or the like, and correct the supply current to the servo motor such that the specified clamping force is obtained.
To correct the errors due to the fabrication and the dynamic element, the servo motor is driven by any one of the supply currents (reference supply currents) listed in the clamping force vs. electric current table obtained from the design data, a pressure detector is clamped between the electrodes of the welding gun, visually read a clamping force value (an actual clamping force) indicated on the clamping force measurer connected to the pressure detector, and, if the actual clamping force is found different from the clamping force value (the reference clamping force) corresponding to the reference supply current selected this time, to be more specific, if the actually measured clamping force is outside the allowance of the reference clamping force, the reference supply current selected this time is corrected by altering the input data by the operator, and the actual clamping force is measured again. By repeating these operations, the actual supply current to the servo motor corresponding to the reference clamping force is obtained. Then, the above-mentioned operations are repeated for all reference supply currents listed in the clamping force vs. electric current table, thereby correcting the clamping force vs. electric current table theoretically obtained from the design data.
To correct the error due to apparatus aging, the same corrective operations as those for the correction of the error due to the fabrication are performed to check if the clamping force listed in the clamping force vs. electric current table is actually applied. The clamping force vs. electric current table is corrected accordingly. If the clamping force value is found significantly different from that at installation of the welding gun, it may be determined that the system including the servo motor and the welding gun is failing.
However, the above-mentioned method of correcting the clamping force vs. electric current table heavily depends on the visual check and manual operation of the operator, requiring many corrective steps and therefore a lot of labor when the clamping force vs. electric current table is corrected for all welding guns. Especially, if the correction of the error due to apparatus aging is to be performed every day at starting the session of the day along with other operational checkout, the operator must bear massive amounts of load and the number of steps. Also, the conventional corrective operation by the operator cannot cope with the recent labor saving trend in fabrication process, especially unattended production.